The other characters in the Highschool DxD world would have a wide range of reactions. The angels might see Optimus as an unknown factor and be a bit suspicious. The fallen angels might try to manipulate the situation to their advantage. And the normal human students would be terrified at first. But as the story unfolds and they see Optimus Prime's noble actions, they could start to look up to him as a hero, much like how the Autobots are seen as heroes in their own universe.
The plot could also have Optimus Prime being mistaken for a powerful new type of devil or other supernatural being at first. As the story progresses, he reveals his true nature and purpose. His interaction with the different factions in Highschool DxD, like the Occult Research Club, could lead to interesting character developments. For example, he might help Rias improve her leadership skills by showing her how he leads the Autobots.
Optimus Prime could fit in as a powerful ally. He would be like a super - powered being in the RWBY world. His presence could shake up the power balance. Maybe he arrives through a portal and immediately starts helping the RWBY teams fight the Grimm. He could be seen as a symbol of hope, much like the Huntsmen and Huntresses are in RWBY.
The Autobots would likely be very surprised at first. Bumblebee might be confused but also very protective. He'd probably try to keep Optimus safe from any potential threats. Ratchet, as the medic, would be both concerned and intrigued. He'd want to make sure Optimus is healthy during the pregnancy.
Optimus Prime crashes on an alien planet during a battle. This planet is inhabited by shape - shifters. At first, they are wary of Optimus, but as they see his noble nature, they decide to help him repair his ship. In return, Optimus helps them defend their planet from an incoming invasion. He teaches them some of his combat strategies and they combine their shape - shifting abilities with his strength, creating a unique and powerful defense force.
The ponies' reactions would vary greatly. Rarity might be fascinated by Optimus Prime's design and might even try to make some sort of fashionable accessory inspired by him. Applejack would be more practical and would wonder what he was doing in Equestria and if he had any good intentions. Optimus would see the ponies as a new and interesting species. He would be patient with their initial wariness of him and would try to show them that he means no harm. He might also be a bit taken aback by their colorful and somewhat chaotic world compared to the more structured world of the Autobots.
At the beginning, they might be cautious around each other. Optimus Prime, being a more traditional Transformer, might find Optimus Primal's more organic and beast - like nature a bit strange. However, as they communicate, they would find common ground in their goals of protecting their people and planets. They could exchange stories about their battles and leadership experiences. For instance, Primal could tell Prime about how he led his Maximals in the Beast Wars, and Prime could share his tales of fighting the Decepticons. This exchange would help them build a bond and understand each other better.
One possible fanfiction could be about Sentinel Prime and Optimus Prime having to work together to save Cybertron from a new, powerful enemy. They might start off with some differences in their approaches but gradually learn to combine their strengths.
Baby Optimus Prime's character can be developed by exploring his relationships. If he has a mentor figure among the Autobots, his interactions with that character can shape him. He might look up to this mentor and try to imitate their good qualities. His relationship with other baby Transformers or younglings can also be important. He could learn about sharing, cooperation, and friendship through these relationships. Additionally, his discovery of his own strength, both physical and mental, can be a key part of his development. For instance, when he first realizes he can lift a small object that others thought was too heavy for him, it gives him confidence.
In a different scenario, Voldemort could be in search of a powerful new weapon and discovers a way to control Optimus Prime. Harry and the Order of the Phoenix have to find a way to break the control and convince Optimus to join their side. They could use their combined powers of magic and technology to defeat Voldemort once and for all.
Writers often develop Optimus Prime as a wise and noble leader. They show his strategic thinking in battles and his moral compass when dealing with humans. For example, he might make decisions that protect both his Autobot comrades and the humans at the same time. Bumblebee, on the other hand, is usually portrayed as more youthful and energetic. His beeping and use of radio snippets to communicate add a unique charm. Writers might have him be more impulsive at times, especially when it comes to protecting his friends, whether they are Autobots or humans.